docs: def read_blob(arg):
      """Read a BLOB from given ``@arg``."""
      result = None
      if arg == '@-':
          result = sys.stdin.read()
      elif any(arg.startswith('@{}://'.format(x)) for x in {'http', 'https', 'ftp', 'file'}):
          if not requests:
              raise error.UserError("You must 'pip install requests' to support @URL arguments.")
          try:
              response = requests.get(arg[1:])
              response.raise_for_status()
              result = response.content
          except requests.RequestException as exc:
              raise error.UserError(str(exc))
      else:
          with open(os.path.expanduser(arg[1:]), 'rb') as handle:
              result = handle.read()
  
      return result